How a Camera Forms an Image
Written by Abin Alex and Anu Joseph.
Abstract
A camera forms an image in seven stages: light, lens, aperture, shutter, sensor, processor and file. This guide sets out those stages and compares them with the human eye, whose design the camera copies although the two systems work differently. It identifies where the comparison breaks down, then examines the effect of sensor size, the difference between DSLR and mirrorless designs, and the place of computational photography. The central point is that the lens forms the optical image, the sensor measures it, and the processor turns the measurement into a usable file. It is written for learners in photography and cinematography.
Keywords: image formation; human eye; lens; sensor; image processor
- How does the human eye form an image?
- How does a camera copy the eye?
- What are the seven stages of image formation in a camera?
- Where does the eye and camera comparison break down?
- What does sensor size change?
- What is the difference between a DSLR and a mirrorless camera?
- What is computational photography?
How does the human eye form an image?
Short answer. Light is bent by the cornea, limited by the iris and pupil, focused by the lens and detected by the retina. The retina turns light into signals that the brain interprets (Atchison, 2023).
- The cornea begins the bending of incoming light. The iris and pupil control how much light enters. The lens fine-tunes the focus on the retina (Atchison, 2023).
- The eye focuses by changing the curvature of its lens. For near objects the lens becomes rounder. For distant objects it becomes flatter (Atchison, 2023).
- The retina holds about 92 million rods for dim light and motion, and about 4.6 million cones for colour and fine detail. Cones are densest at the centre of the fovea (Curcio et al., 1990).
- Signals travel along the optic nerve to the brain. Vision is built by the brain in steps, not simply recorded by the eye (Atchison, 2023).
Cham tipLearn the order: cornea, iris and pupil, lens, retina. Each camera part copies one of them.
Cham taskLook at a friend's eye in dim light, then under a bright lamp. Watch the pupil change size.
Check yourself: Which part of the eye controls how much light enters?
The iris, which changes the size of the pupil.
How does a camera copy the eye?
Short answer. The eye and the camera solve the same problems with different parts. Each camera part does a job that the eye did first (Ray, 2002) (Atchison, 2023).
- Focus: the cornea and lens of the eye match the lens elements of a camera.
- Light control: the iris and pupil match the aperture diaphragm.
- Detection: the retina matches the sensor, and the optic nerve matches the sensor readout circuits.
- Interpretation: the visual cortex matches the image processor.
- The eye has no part that matches the shutter. The eyelid protects the eye, while the shutter sets the exposure time (Ray, 2002).
| Job | In the eye | In the camera |
|---|---|---|
| Focus | Cornea and lens | Lens elements |
| Light control | Iris and pupil | Aperture |
| Exposure time | No direct match | Shutter |
| Detection | Retina | Sensor |
| Signal transfer | Optic nerve | Readout circuits |
| Interpretation | Visual cortex | Image processor |
Table 1. Matching parts of the eye and the camera (Ray, 2002) (Atchison, 2023).
Cham tipUse the table as a memory aid, but remember that similar jobs do not mean identical mechanisms.
Cham taskDraw an eye and a camera side by side. Label which part does each job in Table 1.
Check yourself: Which camera part has no direct match in the eye?
The shutter.
What are the seven stages of image formation in a camera?
Short answer. Light reflects from the scene, the lens bends and focuses it, the aperture controls how much passes, the shutter controls for how long, the sensor turns photons into charge, the processor builds the image and the camera saves a file (Ray, 2002) (Nakamura, 2006).
- Light: the reflected light from the scene is the raw material of every picture.
- Lens: bends and focuses the light to form an optical image.
- Aperture: controls the quantity of light. Shutter: controls the duration of light.
- Sensor: converts photons into electrical charge.
- Processor: reconstructs image data from the sensor signal.
- File: the result is saved as RAW, JPEG or HEIF (Nakamura, 2006).
- The lens forms the optical image, the sensor measures it and the processor produces a usable file (Nakamura, 2006).
Cham tipSay the chain aloud: light, lens, aperture, shutter, sensor, processor, file.
Cham taskWrite the seven stages in order on a card, then add one setting or part that you control at each stage.
Check yourself: Which two stages control the amount and the duration of light?
The aperture controls the amount. The shutter controls the duration.
Where does the eye and camera comparison break down?
Short answer. The comparison helps, but the eye and the camera differ in three ways. The eye adapts over time and gaze, vision is updated continuously and film motion is not an afterimage (Anderson and Anderson, 1993).
- Adaptation: the eye adjusts as you look around. A camera records one fixed exposure. HDR methods bracket or merge frames to get closer.
- Processing: the brain updates vision continuously. A photograph samples one moment, and video samples a sequence of moments.
- Motion: cinema works by temporal integration and apparent motion. It is not simply images blending in the eye (Anderson and Anderson, 1993).
- These differences, more than the similarities, shape exposure choices, HDR, frame rate and motion rendering.
Cham tipWhen a camera picture looks different from what you saw, ask which of the three differences caused it.
Cham taskPhotograph a room with a bright window. Compare the photo with what you saw and note what the camera lost.
Check yourself: Is a photograph the same as what the eye sees?
No. A photograph records one fixed exposure, while vision keeps adapting and updating.
What does sensor size change?
Short answer. Format means the size of the recording surface. A larger sensor records a wider view with the same lens, tends to give shallower depth of field at the same framing and f-number, and collects more total light (Ray, 2002) (Nakamura, 2006).
- Compare the active sensor area first, not the names. Dimensions are written width by height.
- Same lens and position: a smaller sensor records a narrower crop of the same image circle. The lens does not reach farther.
- Same framing and f-number: a larger format usually needs a longer focal length, which gives shallower depth of field.
- Same exposure per unit area: a larger sensor collects more total light. Noise and detail still depend on sensor design, pixel size, lens and processing.
- Sensor size changes optical relationships. It does not guarantee image quality. Always state what is held constant.
- The Camera Sensor guide and the Focal Length guide go further.
| Format | Size (mm) | Area vs full frame |
|---|---|---|
| Digital medium format | 44 × 33 | 1.68× |
| Full frame | 36 × 24 | 1.00× |
| APS-C | 23.6 × 15.7 | 0.43× |
| Micro Four Thirds | 17.3 × 13 | 0.26× |
| 1-inch type | 13.2 × 8.8 | 0.13× |
| Phone (example) | About 9.8 × 7.3 | 0.08× |
Table 2. Common sensor sizes. APS-C, 1-inch type and phone sizes vary by model.
Cham tipWhen comparing cameras, write the sensor size in millimetres, not only the name.
Cham taskFind the exact sensor size of your camera. Work out its area compared with full frame.
Check yourself: Does a smaller sensor make the lens reach farther?
No. It records a narrower crop of the same image.
What is the difference between a DSLR and a mirrorless camera?
Short answer. Both record on a sensor. A DSLR shows an optical view through a mirror and prism. A mirrorless camera shows an electronic preview from the sensor. The viewing path is different, not the image quality (Ray, 2002) (Peres, 2013).
- In a DSLR the lens forms an image that a reflex mirror at 45 degrees sends up through a pentaprism to the optical viewfinder.
- When you press the shutter button, the mirror flips up, the viewfinder goes dark and light reaches the sensor.
- A pentaprism is solid glass and is usually brighter, heavier and costlier. A pentamirror is lighter and cheaper but usually dimmer.
- In a mirrorless camera light reaches the sensor directly. The processor sends a live view to the electronic viewfinder or the rear screen.
- Mirrorless bodies can be smaller because there is no mirror box, and the lens mount sits closer to the sensor.
- Format and camera type are different things. Full frame and APS-C exist in both DSLR and mirrorless cameras.
| Point | DSLR | Mirrorless |
|---|---|---|
| Viewing | Optical, mirror and prism | Electronic, from the sensor |
| Light path | Mirror lifts for exposure | Sensor sees light directly |
| Autofocus | Dedicated phase detect in optical viewing | On-sensor autofocus |
| Body | Deeper mirror box | Shorter flange distance, smaller body |
Table 3. DSLR and mirrorless compared (Ray, 2002) (Peres, 2013).
Cham tipChoose by viewing style, lens range and handling, not by the idea that one type has better quality.
Cham taskLook through a DSLR and a mirrorless camera. Note what each viewfinder shows when you change the exposure.
Check yourself: What does the reflex mirror in a DSLR do when you press the shutter button?
It flips up so that light can reach the sensor.
What is computational photography?
Short answer. Computational photography combines optical capture with calculation. The camera may merge frames, detect subjects and process the data before it saves the file (Lukac, 2011).
- Multi-frame capture: several frames can be aligned and combined to extend dynamic range or to reduce noise (HDR and stacking) (Lukac, 2011).
- Subject recognition: on-sensor systems can detect and track eyes, faces, animals or vehicles where the camera supports it.
- Image pipeline: the processor performs demosaicing, noise reduction and tone mapping on the sensor data (Nakamura, 2006).
- Phones, computers, vehicles and connected devices have made image capture widespread.
- Computation does not replace optics. It combines optical capture with algorithmic reconstruction (Lukac, 2011).
Cham tipKnow what your camera does automatically so that you can switch it off when you need control.
Cham taskShoot the same dim scene with your phone night mode on and off. Compare the noise and the detail.
Check yourself: Does computational photography replace the lens?
No. It adds calculation to the light the lens captures.
Questions beginners ask
How does a camera form an image?
The lens bends light to form an image, the aperture and shutter control how much light and for how long, the sensor measures the light and the processor saves the result as a file.
What are the parts of the eye and the camera that match?
The cornea and lens match the lens elements, the iris matches the aperture, the retina matches the sensor and the visual cortex matches the image processor.
Does the eye have a shutter?
No. The eyelid protects the eye, but nothing in the eye matches the exposure timing of a camera shutter.
Does a bigger sensor always give a better picture?
No. A larger sensor collects more total light, but noise and detail also depend on sensor design, pixel size, the lens and processing.
What is the difference between a DSLR and a mirrorless camera?
A DSLR uses a mirror and prism for an optical view. A mirrorless camera shows an electronic preview from the sensor. Both can use the same sensor sizes.
What is computational photography?
It is the use of calculation, such as merging several frames or detecting subjects, together with optical capture to make the final image.
References
- Atchison, D.A. (2023) Optics of the Human Eye. 2nd edn. Boca Raton: CRC Press.
- Anderson, J. and Anderson, B. (1993) ‘The myth of persistence of vision revisited’, Journal of Film and Video, 45(1), pp. 3–12.
- Curcio, C.A., Sloan, K.R., Kalina, R.E. and Hendrickson, A.E. (1990) ‘Human photoreceptor topography’, The Journal of Comparative Neurology, 292(4), pp. 497–523. doi.org/10.1002/cne.902920402
- Lukac, R. (ed.) (2011) Computational Photography: Methods and Applications. Boca Raton: CRC Press.
- Nakamura, J. (ed.) (2006) Image Sensors and Signal Processing for Digital Still Cameras. Boca Raton: CRC Press.
- Peres, M.R. (ed.) (2013) The Focal Encyclopedia of Photography. 4th edn. Oxford: Focal Press.
- Ray, S.F. (2002) Applied Photographic Optics. 3rd edn. Oxford: Focal Press.
How to cite this guide. Alex, A., & Joseph, A. (2026). How a camera forms an image: A guide for photographers and cinematographers. Creative Hut Academy. https://creativehut.org/library/how-a-camera-forms-an-image/
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